html,
body {
    width: 100%;
    height: 100%;
}

#container {
    width: 100%;
    height: 100%;
    display: flex;
    justify-content: flex-start;
    align-items: stretch;
}

#left {
    flex: 1;
    background-color: green;
}

#right {
    flex: 4;
    background-color: red;
}